1. Field of the Invention
The present invention relates to a foreign particle detecting apparatus and, more particularly, to a foreign particle detecting apparatus for detecting foreign particles or foreign matter (e.g., non-transparent dust) attached to a substrate such as a reticle, a photomask, a semiconductor wafer, or the like used in a semiconductor manufacturing apparatus.
2. Related Background Art
In an IC manufacturing process, an exposure circuit pattern formed on a substrate such as a reticle, a photomask, or the like is transferred onto a wafer surface coated with a resist using a semiconductor printing apparatus (stepper or aligner). In this case, if foreign matter such as dust is present on the substrate surface, the foreign matter is transferred onto the wafer surface together with the circuit pattern, thus decreasing the yield in the manufacture of ICs. For this reason, it is indispensable to detect foreign matter on the substrate in the IC manufacturing process. As described in U.S. Pat. No. 4,468,120, and the like, various detecting methods have been proposed.
FIG. 17 shows a conventional foreign particle detecting apparatus. Coherent light emitted from a laser light source 11 is converted into a parallel light beam having an increased spot size by a beam expander 12. The parallel light beam with the increased spot size is obliquely projected on the surface of a substrate 15 via a scanning mirror 13 and a scanning lens or an f-.theta. lens 14. In this case, the scanning mirror 13 is rotated or vibrated to one-dimensionally scan the beam on the substrate 15 in the x direction. At the same time, a stage (not shown) for mounting the substrate 15 is moved in the y direction. Thus, foreign matter attached onto the substrate surface can be detected over substantially the entire region of the surface of the substrate 15. A plurality of light-receiving elements 166, 167, and 168 are arranged at positions separated from optical paths of regular reflected light and regular transmission light from the substrate. The presence/absence of foreign matter on the substrate 15 is detected using output signals from the plurality of light-receiving elements 166, 167, and 168.
In this apparatus, since the spatial distribution of scattered light from the circuit pattern has strong directivity, output values from the light-receiving elements are extremely different from each other. In contrast to this, since the spatial distribution of scattered light from foreign matter has no directivity, the output signals from the light-receiving elements become almost equal to each other. Therefore, the output values from the light-receiving elements are compared with each other to discriminate foreign matter from the circuit pattern.
However, in recent years, miniaturization of IC circuit patterns leads to miniaturization of circuit patterns on substrates such as reticles or photomasks. As a result, the spatial distribution of scattered light from a circuit pattern becomes isotropic, and tends to have no directivity. In other words, the output values from the light-receiving elements for receiving diffracted light from the circuit pattern tend to be equal to each other.
Therefore, almost no difference among output values from the light-receiving elements is observed between foreign matter and the circuit pattern, and it becomes difficult to discriminate foreign matter from the circuit pattern. In addition, miniaturization of circuit patterns requires an increase in detection resolution.